Psychrometric chart

Psychrometric chart 

This chart contain moist air properties.

Construction

Refer Fig. is a Psychrometric chart.

 

  • Dry bulb temperature is the abscissa (along x-axis) and specific humidity is the ordinate (along y-axis) at 1 atmospheric pressure. It contains the following lines:
  • Vertical lines for dry bulb temperature
  • Horizontal lines towards right for specific humidity
  • Horizontal lines towards left up to saturation curve  for dew point temperature.
  • Relative humidity lines are curves, on the extreme left is 100 % relative humidity line. Then 90 % 80% and so on.
  • Constant enthalpy lines and wet bulb temperatures are slant (diagonal ) lines and coincide.
  • Constant volume lines are also slant lines but are having more slope than the enthalpy lines.
  • Wet bulb temperatures are available on the saturation curve (100 % RH).
  • dew point temperatures are available on the saturation curve (100 % RH
  • Enthalpy lines represent the enthalpy of moist air per kg of dry air.

Psychrometric properties

  1. Dry bulb temperature, tdb
  2. Wet bulb temperature, twb
  3. Dew point temperature, tdp
  4. Specific humidity, w
  5. Relative humidity, Ф
  6. Enthalpy of moist air, h
  7. Specific volume of moist air/kg of dry air

 Utility of the psychrometric chart

When any two of the above properties are known, then the remaining five properties can be determined.

Suppose dry bulb and wet bulb temperatures are given.

Draw a vertical line as per given dry bulb temperature and move along the constant enthalpy line as per given wet bulb temperature. It will meet the dry bulb vertical line. Yhus we get a point on the chart.

(i) To find the dew point temperature, move horizontally towards left till it meets the saturation line.Read the dew point temperature at this point.

(ii) to  find the specific humidity, move horizontally towards right extreme and read the value on the vertical line.

(iii)To find the relative humidity, find the position of the point in between two relative humidity lines and interpolate the required value.

(iv) To find the enthalpy, locate the constant enthalpy line passing through the point. Read its value from the enthalpy values marked on the chart.

(v)To find the specific volume, locate the two specific volume lines in between which the point lies. Interpolate the value of specific volume at the point

Use of psychrometric chart

we can know the condition of air present in a room, varanda outside (covered or uncovered) or anywhere by the use of a sling Psychrometer by measuring the dry and wet bulb temperatures. With the help tdb and twb, first point can be located on the psychrometric chart. We know the condition to be maintained in the conditioned room. Thus another second point can be marked on the Psychrometric chart. Now it is required to reach from first point to second point  in order to make the room air conditioned.This can be achieved with the help of horizontal, vertical lines and moving along the saturated curve. These movements are called psychrometric processes shown in Fig. below.

PSYCHROMETRIC PROCESSES

  • Sensible heating moving horizontally towards right of the point.
  • Sensible cooling moving horizontally towards left of the point.
  • Humidification moving vertically upwards of the point
  • Dehumidification moving vertically downwards of the point
  • Heating and humidification
  • Cooling and dehumidification
  • Cooling and humidification
  • Heating and dehumidification

ADIABATIC MIXING OF TWO AIR STREAMS

In this there are two streams. One is room recirculated air while other is the fresh air. These are mixed either before the cooling coil or after the cooling coil.Let the flow rates of recirculated air be m.re = m.1 and of fresh air be  m.fr = m.2 . Let the mixture condition is point ‘3’.The condition of the mixed air can be found by using the moisture balance and enthalpy balance.

Moisture balance

m.1w1 + m.2w2 = (m.1 + m.2)w3

Energy balance

m.1h1 + m.2h2 = (m.1 + m.2)h3